Microwave detector circuit



Sept. 30, 1958 D, M SHARP 2,854,634

MICROWAVE DETECTOR CIRCUIT Filed May 3, 1956 MICROWAVE DETECTOR CIRCUITDouglas M. Sharp, Allendale, N. J., assgnor to Inter-l nationalTelephone and Telegraph Corporation, Nutley, N. 5., a corporation ofMaryland This invention relates to methods and means for detecting andmeasuring electromagnetic wave energy and more specifically to theutilization for this purpose of a simple circuit arrangement formeasuring power variations in both continuous-wave and modulatedmicrowave signals.

Microwave energy is ordinarily measured by either thermal or electricalmethods. The electrical methods for the detection of microwave energyinvolve rectification of this microwave energy, i. e., conversion to alower frequency by means of a non-linear element, and the detection ofthis energy by ordinary low-frequency techniques. Useful non-linearelectrical devices for accomplishing this demodulation are the germaniumand silicon crystal detectors. These may be of the point-contact orjunction type, and, in general, have good non-linearity response atmicrowave frequencies. However, such crystal detector devices/as nowknown are not operable at substantially elevated ambient temperatures.Furthermore, their power-handling capabilities with respect towithstanding high-peak powers are severely limited. Also, these devicesare frequently erratic and unreliable in use.

Attempts have also been made to use close-spaced vacuum diodes formicrowave detection. Because of the large transit time of the electronsbetween the elements of the tubes, the conversion or rectificationeiciency is relatively low. Attempts to improve this rectificationeciency by a closer spacing of the electrode elements have provenunsatisfactory. The precise tolerances required for this close spacingmake such tubes extremely diicult to construct with any degree ofreliability.

Where the thermal method is used, the high-frequency power propagated isconverted to heat, which is -then measured by a temperature-responsivetechinque. For high-level power, that is, in excess of one watt,measurement of the thermal energy is usually made by a calorimetricmethod, using either water or a gas such as ammonia as the calorimetricduid. For lowand mediumlevel power measurements, various types ofbolometers, principally those such as the barreter and the thermistor,have been used. These latter are temperature-responsive relativelysensitive power detectors which are capable of masuring a few microwattsof power when used in properly designed bridge circuits. The individualcharacteristics of these temperature-sensitive devices vary, some beingrelatively slow acting compared to others; and their over-allsensitivity further depends upon the precision and sensitivity of thevarious complex measuring circuits, such as bridge circuits, in whichthey are used.

One of the difculties existing in obtaining indications of low-powervariations using bolometers is where the microwave signal source to bedetected and measured is a continuous-wave signal. Because of thelimited sensitivity of the bolometer as Well as the associatedcircuitry, which may require precise temperature compensation,relatively small changes in the power level of continuouswave microwavesignals cannot be determined. Either a mechanical chopper to modulatethe continuous-wave signal or a direct-current amplifier must be 'usedin order ted States Patent' Patented Sept. 30, 1958 to detect thelow-level continuous-wave radio-frequency power variations. Such deviceslimit the over-all sensitivity. Noise is introduced, for example, with amechanical chopper, or drift occurs when using a directcurrentamplifier. Furthermore, a circuit used to detect and measure powervariations occurring in modulated microwave signals is generally notequally suitable for use where unmodulated continuous-Wave microwavesignals are to be detected. Also, existing circuitry requires the use ofirnpedance-matching transformers as well as the provision of separatedirect-current biasing sources for the bolometer used. A considerableneed therefore exists for a simple, reliable circuit arrangement fordetecting and measuring microwave signal energy, particularly of thecontinuous-wave type. v

It is an object of the present invention to provide a simple detectorfor microwave energy that is free from the limitations of known devicesas hereinbefore described.

It is a further object to provide such a detector that is equallysuitable for detecting low-power variations in microwave signals whetherof the continuous-wave or modulated type.

It is a feature of this invention that a simple versatile circuitarrangement is provided wherein a bolometer forms part of the biasingarrangement of the cathode circuit of a vacuum tube, thereby eliminatingthe need for a separate direct-current biasing source for the bolometer.

It is a further feature that where the microwave signal impressed on thebolometer is of a continuous-wave nature, means are provided forimpressing an audio signal on the grid of the vacuum tube so that thedirect-current level of the'detected microwave signal, i. e., variationof power therein, is modulated by the audio signal, this modulatedoutput then being in turn readily detected by conventional low-frequencytechniques.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent by reference tothe following description taken in conjunction with the accompanyingdrawings, wherein:

Fig. l is a schematic circuit arrangement of a bolometer for the directreading of power variations in microwave signals whether of thecontinuous-wave or modulated type; and

Fig. 2 is a schematic circuit arrangement of a bolometer in a balancedcircuit for the detection and measurement of microwave signals of thecontinuous-wave and modulated types.

Referring to Fig. l, an electron vacuum tube 1 is shown having at leasta cathode 2 and anode 3, and preferably a grid 4. This tube ispreferably a triode vacuum tube of the indirectly heated type having aseparate heater lament 5. The bolometer 6 is connected in the cathodecircuit of the tube and serves as a cathode bias resistor as well, atthe same time providing degenerative operation to obtain improvedstability. While barreters and thermistors are commonly employed andpreferred as bolometers, other suitable temperature-responsiveresistance devices, such as tungsten filaments, may also be used. Themicrowave signal source 7 is applied to the bolometer as shown in Fig.l. This source may be in the form of a microwave generator or otherappropriate signal source. Where this source is of a modulated type,switch 8 is left open. For this type of operation, the grid circuit iseffectively inoperative. Where the microwave signal source is of acontinuous-wave type, switch 8 is closed, thereby coupling audio signalsource 9 through capacitor 10 to the grid 4, which is biased by variableresistor 11 and direct-current voltage source 12. The resultant signalappearing in the anode circuit will then have an audio frequencycomponent which varies in amplitude according to variations of cathodebias and there- .3 fore of the amplification factor, as determined byany direct-current change across the bolometer resulting from a powerchange in the continuous-wave microwave signal. This will appear .acrossresistor 13-:and-be coupled throughcapacitor k14 to alternating-current.amplier 15, rectifier 16 and indicator 17. `Resistor y18 .and capacitor19 .arepreferably selected to have arrelatively long time constant andtherebyprotect vthe :bolometerfrorn powersupply surges. In use, thevalue of grid biasing resistor 11 would be vadjusted with nomicrowavesignal or .audio signal present inorder to have the `anode current agreewith the bolometerbias specifications. The .above circuit wouldbeessentially .of the .direct-reading type, 4and the-amplier 15 may .be.tuned .for desired frequencies in yorderto improve sensitivity yandgain.

In Eig. 2 is shownfatbalanced circuit utilizing the .same @principles ofdetection and ameasurement as described in conjunction with Fig. 1.Thus, the bolometeris similarly `connected in the cathodecircuit,thereby obviating-the 'need for a separate biasingsource for thebolometer and also providing degenerativeoperation to .obtain improvedstability. Corresponding circuit :components .in .fFig. 2

vhave been .numbered to correspond .to the numbering .of Eig. 1.Themethod of operatin-.of..this'.circuituwill .be understood from theoperation describedforfFig. `1. The

'vacuum tube 1.having a cathode, anode and-grid is :con-

nected in a iirst branch 20 of this balanced circuit zin series withbolometer .6', whichzisconnected between .the

cathode .2' and ground. `A .second :vacuum tube 22 .is connected in asecondbranchof lthis `circuit inseries with 1a variable resistance 24,which is '.connectedzbetweenrthe cathode 26 and .ground. :Both anodes ofthe :two tubes are connected together at one junction .of the twobranches, v'and resistance24 and bolometer 6' are connected .togetherfand to .ground at the other junction. Signals :from audio signal source9 are .coupledthrough transformer .28 :to grids 4' and 127 of tubes11F-and 22. Thecenter taptof transformer 28 is coupled to .groundthrough -a 4con- '.denser `29. It :should 'be lnoted -that the laudiosignal :pends upon the setting .of resistance Z4. Since the signals fromthe two branches are of opposite phase, the .difference in amplitudeiszapplied to the detector circuit lconnected to 'the junctions of thebranches, to thereby measure the amplitude of the microwave signal.

Such a balanced circuit provides somewhatmore sensi- .ftivity than canbe obtained with the circuit illustrated in Fig. l. Both of thecircuitsillustrate'din Figs. l and 2 are equally versatile in beingcapable of use with both :continuous-wave .or modulated microwave signalsources.

In Table lis shown the manner in which the circuits of Figs. l and 2respond to changes in themicrowavepower level, determined for conditionswhere a continuous-wave I( C. W.) microwave signalisimpressed across thebolom- -eter .and an articial audio signal of fixed amplitude is -usedfor v modulating purposes.

Meter Indication vSignal 'Conditions Fig. 1 Circuit Fig. 2 Circuit No`C.'W. Microwave Signal, Tired Audio Signal Added.

SmallC. W. 'Microwave SignalfFixedAudio' Sig- .nal Added.

"barge C. "W. Microwave FSignaljFixed 'Audio Signal Added.

Maximum Reading- Zero' Reading.

Lower Reading.. Increased' Reading.

Lowest`Reading. Maximum Reading.

As may be seen from the foregoing description, the circuit arrangementsprovided in Figs. 1 and 2 are considerably simpler than those utilizedheretofore, namely, impedance-matching transformers and separatedirectcurrent bias for the bolometer have been eliminated, and,furthermore, the bolometer'may-be used to detect changes in power levelfor both continuous-wave and modulated microwave-signalrsources. Whilethis invention has been illustrated by the use of an electron vacuumtube, other multielectrode devices may be used, such as point-contactand junction transistors and gas discharge tubes, with appropriatecircuit modifications where required. The use ofthe bolometer'to serveas the biasing source for the multielectrode device aswell as use of oneelectrode on which an audio signal is impressed for modulating themicrowave signal is considered characteristic of transistorampliiierdevices. Asfillustrated herein, however, this in- Mention iinds'itspreferred use with a triode-type vacuum tube. The circuitv arrangementsdescribed lind their greatest utility Vfor the .detection andmeasurement of low-power microwave signals, namely, those below l0milliwatts; they may equally well be used for medium-power detection,from 10.*mil1iwatts to .l watt, and for high-power detection, .about .lwatt. Basically, the limitations irn- .posed ony the power peaks handledby thedevice are those `presented bythefpower-.handling .capabilities ofthe multielectrode 4deviceand .the bolometer. While the systemdescribedis capableofacting asa microwave detector at a frequency rangeof -.10U-to 20,000 megacycles, .a preferredrangeis between 1,000 .and12,000 megacycles.

While lfhave described above'the principles of my invention .inVconnection With specific circuit arrangements, it is to .beclearlyunderstood'that this description is made 'only 4by way .of example .andnot as .a limitation to the scope of my invention* as .set .forth in theobjects thereof and. in the accompanying claims.

lzclaim:

1. A system for detecting electromagnetic `wave energy comprisingabalancedcircuit including a firstand second branchfeach connected.between airst and a second junction '.point, 1said1frst branchcomprising a first electron tube having a cathode, anode and gridinseries with a bolometerconnected.between said cathode and said sec-'ondljunctionrpoin said second branch comprising a second electron tubevhaving a cathode, anode and grid in vseries iwith a variable`resistance connected between the cathode vof;said :second ftube andsaid second junction point, .theanodeiof :each saidtube beingconnectedto the said rst -junction point, :means for impressing a microwavesignal '.on said bolometer, .an audio signal source, said .audio .signalsource .being coupled to each of said gridsto applvan audio -signalofoppositefphase and substantially equal in amplitude with respect to saidsecond junction point to each said grid, whereby variations in saidmicrowave signal produce variations of net audio signal .outputfromsaidbalanced circuit, and a vdetector circuitconnected between said irstand second junction points for detecting the amplitude of said net audiosignal output.

12. Asystem for detecting:electromagnetic-wave energy comprisingamultielec'trodedevice, aireference potential, bias means coupledbetween said referencepotential and one-.of saidrelectrodes at afpredeterminedixed value, a bolometer coupledbetween said referencepotential and another of said electrodes to establish a bias for saidanother Aof said electrodes, lmeans for'impressing a microwave signalon.said bolometer to varythe fbias of said anothertof-.said electrodes inaccordancefwith power level variationsof said microwave `signal,.anexternalaudio signal source having .ya Xed .amplitude output, selectivemeans -to .couple `the audio :signal of said audio signal sourceat willtosaidone-of said electrodes, the variations of .the 'bias1of saidanother .of-said electrodes amplitude modulating :said :audio fsignal,and .means .coupled to a l.third electrode :of .said electrodes fordetecting the audio signal output level to thereby measure the level ofsaid microwave signal.

3. A system for detecting electromagnetic wave energy comprising anelectron tube having a cathode, anode and grid, a reference potential,bias means coupled between said reference potential and said grid ofsaid electron tube at a predetermined xed value, a bolometer coupledbetween said reference potential and said cathode of said electron tubeto establish a bias for said cathode, means for impressing a microwavesignal on said bolometer to vary the bias of said cathode in accordancewith power level variations of said microwave signal, an external audiosignal source having a xed amplitude output, selective means to couplethe audio signal of said audio signal source at will to said grid, thevariations of the bias of said cathode amplitude modulating said audiosignal, and means coupled to the anode of said electron tube fordetecting the audio signal output level to thereby measure the level ofsaid microwave signal.

4. A system for measuring continuous wave radiofrequency powercomprising an electron tube having a cathode, anode and grid, areference potential, bias means coupled between said reference potentialand said grid of said electron tube at a predetermined xed value, abolometer coupled between said reference potential and said cathode ofsaid electron tube to establish a bias for said cathode, means forimpressing a continuous wave microwave signal on said bolometer to varythe bias of said cathode in accordance with power level variations ofsaid microwave signal, an external audio signal source having a xedamplitude output connected to said grid to couple an audio signalthereon, the variations of the bias of said cathode amplitude modulatingsaid audio signal., and means coupled to the anode of said electron tubefor detecting the audio signal output level to thereby measure the levelof said continuous wave microwave signal.

References Cited in the lile of this patent Article by B. P. Hand,published in Hewlett Packard Journal, vol. 1, No. 9, May 1950. Copyavailable in 324-95.

